The present invention relates to a novel and useful elastomeric copolymer of vinylidene fluoride and at least one other fluorine-containing monomer, and more particularly to fluorine-containing copolymers comprising a a copolymer produced by copolymerizing the above monomers in the presence of a water-soluble radical initiator at a first stage and a copolymer produced by copolymerizing the above monomers in the presence of an oil-soluble radical initiator at a second stage, and to a process for preparing the same and a composition of the same.
Fluorine-containing elastomeric copolymers such as vinylidene fluoride-hexafluoropropene copolymer and vinylidene fluoride-hexafluoropropene-tetrafluoroethylene terpolymer have excellent heat stability, oil resistance, solvent resistance and chemical resistance, and have been widely employed as industrial materials.
Hitherto, fluorine-containing elastomeric copolymers are generally prepared by emulsion polymerization of monomers in an aqueous medium in the presence of a water-soluble radical polymerization initiator such as inorganic peroxide compound or a combination of an inorganic peroxide compound with a reducing agent, or by solution polymerization of monomers in an organic solvent in the presence of an oil-soluble polymerization initiator such as an oil-soluble peroxide compound. Also, there is proposed in U.S. Pat. No. 3,801,552 a process for preparing fluorine-containing elastomeric copolymers by suspension-polymerizing monomers in an aqueous medium containing an organic solvent in the presence of an oil-soluble radical initiator.
However, fluorine-containing elastomeric copolymers prepared by the emulsion polymerization using an inorganic radical initiator have the disadvantage that when it is cured by a curing system aiming at providing a cured rubber having a low compression set, for instance, by a curing system consisting of a curing agent of an aromatic polyhydroxy compound and an accelerator, the curing reactivity is generally poor. Therefore, when curing the copolymer by using a curing agent and accelerator in a small amount sufficient for giving cured rubbers having a desirable low compession set, the cure rate is slow. On the other hand, when curing the copolymer by using a curing agent and accelerator in a large amount required for curing it at satisfactory cure rate, the cured rubber has undesirable high compression set and also the elongation is lowered. Fluorine-containing elastomeric copolymers prepared by the suspension polymerization using an oil-soluble radical polymerization initiator overcome to some extent the defect of the elastomeric copolymers prepared by the emulsion polymerization, and can provide cured rubbers having a low compression set at a desirable cure rate. However, such a suspension polymerization process has the disadvantages on preparation steps that the conversion of monomers is low and the recovery of the employed organic solvent is required, and is not desirable in practical use.